enhancement (optional): record/replay option for hermetic integration tests

[maximn]

From the AI-native docs audit (B9 in .agents/known-issues.md).

Context
Integration tests hit the live Google APIs (no cassette/VCR), so they require a real GOOGLE_API_KEY, network access, and available quota, and can fail for reasons unrelated to a change. This is a deliberate, documented choice (see .agents/testing.md) — filing as an optional enhancement, not a defect.

Possible
Add an opt-in record/replay layer (e.g. a caching HttpMessageHandler) so CI can run a hermetic subset while live tests remain available on demand.

Severity: low / optional.

[maximn]

Status: the VCR layer this asks for already landed in #306 — what's left is the cassette dataset.

The opt-in record/replay harness is implemented and merged: GoogleMapsApi.Test/Vcr/ provides a VcrDelegatingHandler with replay (default) / record / auto / live modes, key+signature redaction, base64 bodies (JSON + binary), per-test cassettes, billable-test gating, and CI wiring (offline replay by default plus a scheduled live drift-check). BaseTestIntegration already builds the test HttpClient around it, so dotnet test defaults to hermetic replay.

Remaining gap: no cassettes are committed yet (GoogleMapsApi.Test/Cassettes/ has only a README), so replay is currently dormant — CI's replay step is cassette-guarded and skips with a warning, and a local VCR_MODE=replay run fails loudly on the integration suite. Seeding the dataset needs a live key and incurs billable charges:

VCR_MODE=record GOOGLE_API_KEY=<key> RUN_BILLABLE_TESTS=true dotnet test

Separately, I corrected the .agents/ knowledge base, which still described the pre-VCR world: the README.md fast-fact now says integration tests default to VCR replay, and the B9 known-issues entry now points at the real residual (harness exists, cassettes not yet recorded) instead of "no VCR exists." Going out as a small docs PR.

Keeping this open to track recording + committing the cassette dataset, then flipping CI's replay step from cassette-guarded to required.